A ramjet is a type of air breathing propulsive engine that uses fixed components to compress and accelerate intake air by ram effect. It is also affectionately known as the flying stovepipe.
FIG. 1 shows the basic elements of a ramjet power cycle and flowpath in accordance with the prior art. Conventional ramjet theory evolved during the early twentieth century and has promised many benefits over other propulsion theories. Conventional ramjets are described by the Brayton power cycle (see W. H. Heiser and D. T. Pratt, et al, Hypersonic Airbreathing Propulsion, American Institute of Aeronautics and Astronautics, Inc. Washington, D.C. 20024 USA, 1994, the contents of which are hereby incorporated by reference). The ideal open Brayton cycle describes the thermodynamic power cycle for the ideal ramjet engine.
In its simplest configuration, the open cycle has three phases. Initially air is taken from the environment and is isentropically compressed, meaning the compression is both adiabatic and reversible; no energy is added to the flow, and no energy losses occur due to friction or dissipative effects. Heat is then added to increase the temperature and energy of the air isobarically, and the air is expanded isentropically into the environment to the initial pressure of the air. The energy released during final expansion is partially consumed by the initial compression. The net work/power done by the cycle on the environment is positive when the isentropic compression results in an internal air total pressure greater than the external air total pressure.
According to Zucrow, Waltrup, and Fry, the first workable ramjet concept was published early in the twentieth century. (see Zucrow, Maurice Joseph, Aircraft and Missile Propulsion, Volume II, The Gas Turbine Power Plant, the Turboprop, Turbojet, Ramjet, and Rocket Engines. New York, John Wiley & Sons, Inc., 1958; Waltrup, et al, “History of U.S. Navy Ramjet, Scramjet, and Mixed-Cycle Propulsion Development,” Journal of Propulsion and Power, Vol. 18, No. 1, January-February 2002, pp. 14-27, American Institute of Aeronautics and Astronautics, Inc., Washington, D.C. 20024 USA; and Fry, Ronald S., “A Century of Ramjet Propulsion Technology Evolution,” Journal of Propulsion and Power, Vol. 20, No. 1, January-February 2004, pp. 27-58, American Institute of Aeronautics and Astronautics, Inc., Washington, D.C. 20024 USA, the contents of which are hereby incorporated by reference.)
In 1928 a German patent (#554,906) was issued to Albert Fono for a ramjet optimized for supersonic flight. Referring back to FIG. 1, the simplest configuration was a duct which included, in order of flow, a diverging inlet/diffuser 101, a constant-area subsonic combustor 102, and a converging-diverging nozzle 103.
FIG. 2 is a block diagram representation of the Fono engine. This configuration reflects the ideal open Brayton cycle's phases respectively. Air enters the diverging-wall subsonic diffuser 201 and passes into the constant-area mixer 202 where it is mixed with the fuel supplied from a fluid reservoir 203. The air/fuel combination is then combusted by a flameholder/igniter in the combustion chamber 204. The combustion products move through a converging-wall subsonic nozzle 205 and then are released into the atmosphere through a diverging-wall supersonic nozzle 206.
For a ramjet engine based upon the ideal open Brayton cycle, an internal air total pressure ratio above unity causes ramjet power. The term “ramjet” comes from the creation of power from the ramming of air into the engine inlet sufficiently to cause internal air total pressure ratio above unity in the combustor and then expelling that higher-pressure air as a jet with propulsive force. Maximum internal air total pressure ratio is achieved when the ramjet's inlet has a sonic condition. However, the Fono ramjet has no direct means for completing the Brayton cycle and creating sonic inlet conditions.
The open Brayton cycle is complete only when the energy released during the final expansion phase is consumed in part to power the initial compression. The Fono ramjet accomplishes the energy transfer from the final expansion to the initial compression indirectly by the synergy of the propulsive force causing the ramming of air into the inlet during sonic forward flight. This indirect energy transfer describes the ramjet conundrum: for the Fono ramjet to be generating thrust it must by moving forward and to move forward it must be generating thrust.
The Fono ramjet conundrum prevents this propulsion concept from operating statically (unmoving) which is an obviously severe limitation for reducing to practice. All known working examples of ramjets are accelerated to critical operating speed (typically Mach 1.0) by an ancillary propulsion engine before generating power. Typically, this is achieved using other propulsion methods incorporated into the body of the aircraft or engine such as solid or liquid rockets or turbojet engines. The simplicity of the design and construction of the Fono ramjet is violated by the need for an ancillary propulsion engine.
To statically start and operate a ramjet engine without an ancillary propulsion system requires the artificial creation of sonic conditions at the inlet of the ramjet. This can be accomplished using a pumping ejector coupled to the inlet of the ramjet. The pumping ejector creates a mixed fluid flow that can achieve sonic speed at the ramjet inlet.
The efficiency of such a pumping ejector can be enhanced by recycling thermal energy from the combustion chamber to the ejector by means of a superheater.